VT Header dropmark appears even if dragged column won't change position

[sanjayssk]

This can be verified in Advanced Demo "Hide and show nodes." In the lower tree, move column 2 to the next column, column 3. The drop mark appears to the left of column 3 indicating that column 2 will be dropped on the left. But it's already on the left so this drop mark is unnecessary. Ditto for moving to previous column, column 1 where the drop mark on the right appears unnecessarily.

Needs a fix in the code where left or right drop mark image is selected. Positions of the two columns have to be compared and the image switched off if the above conditions are met.